Abnormal X chromosomes in man: origin, behavior and effects.

Abstract

Abnormal human X chromosomes, their origin, phenotypic effects, and especially their inactivation are reviewed. In cases of balanced reciprocal X-autosomal translocations (Table 2), almost always the normal X is inactivated. Most of such patients suffer from gonadal dysgenesis, which might be caused either by functional hemizygosity for a recessive gene or by a position effect resulting from a rearrangement involving a certain region of Xq. In cases with 46 chromosomes and an unbalanced X-X translocation, the translocation chromosome is inactivated; in most such patients, an X0 cell line is also present (Table 3). Some of the possible modes of origin of such translocations are presented in Fig. 1; these also explain the occurrence of the X0 cell lines. In patients with 46 chromosomes and an unbalanced X-autosomal translocation, the whole translocation chromosome seems to be inactivated, if the autosomal segment is attached to Xp. If on the other hand the segment is, attached to Xq, inactivation seems to be limited to the X part (Fig. 3).